A group of pure - blood wizards who believe that magical creatures are inferior and should be enslaved could be the antagonists. They oppose the idea of respecting and co - existing with creatures, and try to disrupt any attempts at harmony between wizards and creatures. This would create a conflict not just between good and evil, but also between different ideologies within the wizarding world.
It could be a dark wizard who wants to use the power of magical creatures for his own evil purposes. He might be trying to control them, or use their parts for dark magic spells. For example, he could be after the horns of unicorns to make a potion that gives him immortality.
It could be a dark wizard who has mastered the art of domination spells. He might be someone who was once a student at Hogwarts but turned to the dark side due to his thirst for power. He uses his skills to control other wizards and witches, and his goal is to rule over the entire wizarding world.
The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.
It could be a new dark wizard who has a vendetta against Harry. This wizard might use a spell to stop Harry from eating, thinking it will weaken him before a final showdown. He could be someone who was once a follower of Voldemort and now wants to take his place and prove himself by getting rid of Harry.
Maybe an ancient evil spirit that was once bound but is now trying to break free by using the summoner as a conduit. This spirit could be manipulating events from the shadows, causing chaos in the wizarding world to draw the summoner out.
It could also be an existing character from the Harry Potter series like Hagrid. Given his love for all creatures, including some that are considered dangerous, he could be at the center of a story about the 'Lord of Dark Creatures'. He might be trying to protect these creatures from those who would use them for evil purposes or trying to rehabilitate them.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
In an alternate dimension, the main antagonist could be a dark entity that has emerged from the very fabric of the dementions themselves. This entity could be using the Dementors as its minions, controlling them to do its bidding. It would be a powerful and mysterious foe, and Harry would have to discover its true nature and weaknesses in order to defeat it.
There is also a possibility that the main antagonist is the Goblet itself if it has been corrupted by some ancient dark magic. It could be summoning Harry not for a normal Tournament but to sacrifice him or use his power for some evil purpose, making the Goblet the force that Harry has to fight against.
Maybe the demon's soul itself could be the main antagonist in a way. It has its own will and desires, which are mainly to spread chaos and destruction. It could possess other characters in the story and make them act against Harry. For instance, it might possess a Hogwarts professor who then tries to undermine Harry's efforts to stop it. The demon's soul's ability to manipulate and corrupt would make it a very dangerous and menacing antagonist.
It could be a rogue Death Eater who wants to take over Voldemort's position and sees Harry and Bellatrix's new - found alliance as a threat. For example, a Death Eater who was always jealous of Bellatrix's closeness to Voldemort.